The endodontic access cavity is the cavity prepared in the crown of the tooth through which root canal therapy is performed. The establishment of access is the first step in the mechanical phase of endodontics. Also, it is the procedure upon which the rest of endodontic therapy rests. No step of the therapy can be omitted without affecting the entire treatment. Penetration into the pulp chamber is the first step, followed by funneling of the preparation so that the access cavity is smallest at the orifice of the canal and largest at the occlusal opening. The penetration is usually accomplished at the occlusal opening, accomplished with a round carbide bur. The funneling step is done after the roof of the penetrated pulp chamber is peeled back and up, by a withdrawal stroke of the round bur. The round bur or a tapered carbide or diamond is then used to flare or funnel the preparation providing straight access and a positive seat for a filling. Such a straight line access is important for adequate canal penetration without obstructions, which also eliminates overhangs. A great many root canal cases fail because the dentist does not obtain adequate access to the canal. Inadequate access makes proper instrumentation and filling of the canal very difficult. A set of general rules for making occlusal openings for endodontic work can be stated. (1) The opening must extend to the full periphery of the pulp chamber including the pulp horns; (2) direct access to the canal must be obtainable; (3) no overhanging portions of the roof of the pulp chamber should remain which may trap pulp debris and blood; (4) destruction of tooth structure should be avoided. The first requirement is important because a frequent cause of discoloration of pulpless teeth is the trapping of blood and organic debris in the pulp horns. Without adequate access, no canal can be properly filed and filled. Failure to remove all the overhanging denting above the pulp chamber may result in retention of large amounts of blood and organic debris in posterior teeth and lead to residual infection. Excessive destruction of tooth structure may result in fracture of the tooth.
Previously, the dentist used a round bur to form an access and might have tried to use this same bur to funnel the canal. However, this causes grooving of the walls of the pulp chamber and yields incomplete debris removal, which results in a rough inner surface that easily harbors debris and tissue. The dentist also made his penetration with a fissure type bur that funnels and smooths the chamber wall, but tends to cut the floor of the pulp chamber, making it difficult to find the canal openings. A more recent technique involves making an access with a round bur along the floor of the chamber, then, using this bur, strip away the roof. A tapered fissure bur is then used to smooth and funnel the circumferential walls. Thus, the recent technique is a two instrument two step procedure. It is apparent that this technique is more time consuming and more costly than a technique using one instrument for both steps. A diamond carbide combination bur has been developed, but the tip, carbide, wore at a different rate than the diamond and it is not properly sized for root canal work.
All presently available diamond burs differ from the present invention. None has a sufficiently rounded end, nor has an adequate amount of diamond coating up the instrument shaft to allow a dentist to fully prepare the access. Prior art burs are diamond coated approximately 6 mm along the shaft. The present invention concerns a diamond coating of at least 10 mm up the shaft. The round end is the usual equivalent of a #2, #4 and #6 round bur, approximately 1/2-1 mm in diameter.